In recent years, a variety of wastewater treatment technologies have emerged, which use chemical, physical chemical, and biological methods to remove contaminants from wastewater.
Membrane water filtration technology has been increasingly used in contaminant removal systems due to its high efficiency, relatively low energy usage, simplified equipment and ease of operation. New membranes are the focus of continuous R&D efforts, such as ultra-filtration membranes, microfiltration membranes, nano-filtration membranes, ceramic membranes and ion-exchange membranes. These membranes may not only fully recover valuable resources in wastewater, but also advance the depth of wastewater treatment with significant environmental and economic benefits. However, the complexity of the manufacturing process, their relatively high price, and possibility of secondary contamination limits the widespread applications of membrane separation technology in wastewater treatment. Furthermore, at present, membranes with dual functions of filtration (of water impurities) and adsorption (of heavy metal ions) remain untapped.
The membranes used in wastewater treatment have to withstand rather harsh chemical conditions. Materials such as polytetrafluoroethylene (PTFE) for example, exhibit good resistance to chemical corrosion and are hydrophobic, are insulating, insoluble and exhibit non-adhesive or “anti-sticking” properties. PTFE films have been mainly used in gas-solid separation, dust collection, air purification, and as porous waterproof coatings on clothing. However, because of their strong hydrophobicity, PTFE films cannot be used in water filtration. Therefore, to produce PTFE composite membranes for use in water filtration, the PTFE films are typically subjected to plasma processing followed by grafting modification of hydrophilic functional monomers. The PTFE films can also be irradiated and compounded together with hydrophilic polymers. However, these processes are both complex and costly. Furthermore, plasma treatment of the PTFE films may undermine the internal structure of the membrane and decrease the membrane mechanical strength.
Another known process for producing composite membranes for use in water filtration employs electro-polymerization to polymerize hydrophilic monomers onto a non-conductive polymer support. However, due to the non-conductive nature of such polymer supports, the rate of electro-polymerization is comparatively lower than that of chemical polymerization. Furthermore, the extent of electro-polymerization is limited by the size of the electrode used. Therefore, such processes are difficult to scale-up and have limited industrial applications.